Improve tax admin page

Improves the layout of the taxes admin page tab. 

![Google ChromeScreenSnapz016](https://f.cloud.github.com/assets/1079437/400382/a38d87ec-a8a0-11e2-8e96-f543fdf81350.png)

Adds new i18n strings. 

I'll be honest and say this isn't as good as it could be. I feel like the tax rates and tax bands fields could look more like the Checkout Form Fields table, or at least like the Custom Fields metabox.